1. Field of the Invention
The present invention relates to a method for evaluating a semiconductor wafer, and more particularly to a method for evaluating a semiconductor wafer by measuring a junction leakage currents.
2. Description of the Related Art
With miniaturization and advance in performance of a memory, a solid-state image pickup device such as a CCD and the like, to improve product yields, enhancement of quality of a silicon wafer as a material has been demanded, and various kinds of silicon wafers coping with such a demand have been developed. In particular, crystallinity of a wafer surface portion that is considered to directly exert its influence on product characteristics is important and, as countermeasures, 1) a high-temperature treatment that is performed in an atmosphere containing an inert gas or hydrogen, 2) reduction of grown-in defects by improving pulling conditions, and 3) an epitaxial growth wafer or the like are developed.
As a conventional method for evaluating electrical characteristics of silicon wafer surface quality, oxide dielectric breakdown voltage (GOI) evaluation has been used. According to this evaluation, a gate oxide film is formed on a silicon wafer surface by thermal oxidation, electrical stress is applied to a silicon oxide film as an insulator by forming an electrode on the gate oxide film, and silicon surface quality is evaluated based on a degree of insulation. That is, if defects or metal impurities are present on the original silicon wafer surface, they are taken into the silicon oxide film by the thermal oxidation, or an oxide film according to a surface shape is formed, and a non-uniform insulator is produced, thus degrading insulation properties. This is reliability of the gate oxide film of an MOSFET in an actual device, and various wafers have been developed for improvement of the above insulation properties. However, even if the GOI has no problem, a reduction in device yield can naturally happen, but such phenomena often occur in recent years with advance in integration of devices in particular.
Among others, in a solid-state image pickup device, for example, when a reduction in, e.g., dark current and an improvement in sensitivity are considered, reducing a junction leakage current caused due to a wafer leads to a decrease in dark current, which eventually contributes to an improvement in device characteristics. When metal contamination is a main cause in particular, with advance in performance of device in recent years, small amount of metal contamination has exerted an influence. On the other hand, in chemical analysis, although an approach for high sensitivity has enabled detection of various kinds of metals, as matters now stand, it is very difficult to grasp which metal in metallic elements detected by the chemical analysis considerably exerts its influence on an actual device or junction leakage.
As a method for identifying such a contamination metal that exerts its influence on the junction leakage, as described in Patent Literature 1, the contamination metal can be identified from temperature dependence characteristics of junction leakage currents.